The semaphore or optical telegraph is an apparatus for conveying information by means of visual signals, with towers with pivoting blades or paddles, shutters, in a matrix, or hand-held flags etc. Information is encoded by the position of the mechanical elements; it is read when the blade or flag is in a fixed position. In modern usage it refers to a system of signaling using two handheld flags. Other forms of optical telegraphy include ship flags, Aldis lamps, and Heliographs.

Semaphore lines preceded the electrical telegraph. They were faster than post riders for bringing a message over long distances, but far more expensive and less private than the electrical telegraph lines which would replace them. The distance that an optical telegraph can bridge is limited by geography and weather, thus in practical use, most optical telegraphs used lines of relay stations to bridge longer distances.

History

Although passing mention of this idea had been made at many points in history, it was apparently the English scientist Robert Hooke who first gave a vivid and comprehensive outline of visual telegraphy to the Royal Society in a submission dated 1684; in it he outlined many practical details, but his system was never put into practice. Over a hundred years later a French engineer, Claude Chappe and his brothers took up the challenge again and succeeded to cover France with a network of 556 stations stretching a total distance of 4,800 kilometres. It was used for military and national communications until the 1850s.

Many national services adopted signaling systems different from the Chappe system. For example, Britain and Sweden adopted systems of shuttered panels (in contradiction to the Chappe brothers' contention that angled rods are more visible). In Spain, the engineer Agustín de Betancourt developed his own system which was adopted by that state. This system was considered by many experts in Europe better than Chappe's, even in France.